An off-premises cable television (CATV) system generally comprises a headend for transmitting television programs with a broadband television signal including regular and premium channels over a cable plant to a multiplicity of subscribers. The cable plant can include a number of trunk lines from which a plurality of feeder lines fan out over a service area. From each feeder line, there are a large number of taps for subscriber drops to individual premises. At each of the taps or along the subscriber drops are interdiction apparatus which inject a jamming signal into those premium or restricted channels that the particular subscriber is not authorized to view. An advantageous interdiction CATV system of this type is more fully set forth in U.S. Pat. No. 4,912,760 by West, et al., the disclosure of which is herein incorporated by reference.
Because of the cable plant characteristics, during transmission the broadband television signal is nonlinearly attenuated, specifically, more at high frequencies than at low frequencies. This nonlinear attenuation as a function of frequency is typically termed slope in the CATV industry. Therefore, to correct this condition, slope compensation is provided in the transmission system by circuits which are frequency dependent in the opposite direction of the cable characteristic thereby cancelling the effect. This correction is normally provided as a function of frequency with a positive slope. However, not all frequency correction is necessarily in the positive direction. During the transmission phase of the signal, transmission amplifiers may add frequency dependent gain so that as the signal travels down the distribution plant it will droop with frequency less rapidly. Therefore, it is not always easy to determine what the slope of the signal will be at a subscriber drop so that a fixed correction may be applied.
To be of advantage in an off-premises site, a slope network must then be prepared to correct for either positive, negative, or zero slope (it must not introduce a correction where none is needed).
Normally, a slope correction network is inserted serially in the path of the broadband television signal. This raises concerns about the return loss of the circuit at the input side and the output side. If the return loss can be maximized, the network will minimize artifacts in the transmitted picture resulting from reflected power. It is known that the manner in which to maximize return loss is to match the output impedance of the previous circuit with the input impedance of the slope network and match the output impedance of the slope network to the input impedance of the next circuit. However, to maximize return loss while providing a flexible frequency dependent circuit with positive and negative slope correction has in the past proven difficult and relatively expensive.
Prior slope compensation networks measured the absolute level of two pilot signals of the broadband signal to determine the amount of correction. A pilot frequency on one end of the band was measured and utilized for slope correction and a pilot frequency at the other end of the band was measured and utilized for gain correction. This circuit requires relatively expensive band pass filters for the measurement of the pilot frequencies and further that the filters be set to detect frequencies which are always present. While this is not too burdensome when talking about trunk amplifiers where the cost per unit can be large because there are so few, this becomes much more of a problem when providing a slope correction network for interdiction apparatus because of the number of units. Even more burdensome is this problem when individual subscriber interdiction units are utilized.
Another problem with the prior slope correction networks is that they usually have two control terminals. This contemplates that the levels from the pilot frequency detectors are at the same levels and that components in the detection circuits do not age at different rates. The dual control inputs also make the networks more complex and costly than need be thereby prohibiting their use for cost reasons in the subscriber sections of interdiction devices.